Many sensor modalities have been developed to sense the presence of water in its different phases. A majority of such sensors were geared to measure humidity (water vapor) levels. Some examples of such sensors include mechanical hygrometers, animal hairs, psychrometers, etc. These sensors have had broad applications across agriculture, weather prediction, medicine, etc.
The introduction of electronics has allowed the miniaturization of humidity sensors and thus is more versatile in terms of deployment. Partly due to this increase in versatility, most of the currently used humidity sensors are electronically based. Electronic humidity sensors measure changes in electron transduction due to air humidity. Generally, these sensors can be categorized into capacitative, resistive, and gravimetric sensors.
Fiber Optic Water Sensors:
While the integration of electronic technologies allowed the miniaturization of humidity sensors, the development of fiber optics (FO) has opened new doors in regards to the precision, operation, low cost, and networking. FO-based sensors also have a unique resistance to water-related damage, in contrast to most commercially available electronic sensors, which may be vulnerable to circuit shorting due to contact with liquid moisture. A wide variety of fiber optical humidity sensors have already been produced, and can be divided broadly into spectroscopic, fluorescent, interferometric, and in-fiber grating sensors.
Spectroscopic FO sensors generally measure changes in the spectroscopic absorption of a humidity-sensitive chemical (e.g. cobalt chloride, Rhodamine B, crystal violet, etc.) arranged on a thin film. Spectroscopic FO sensors can have a wide humidity range and fast response times (e.g a range between 0 to 95% relative humidity and response within 2 minutes). Fluorescent based FO sensors measure the fluorescence of certain dyes due to photonic excitation. For example, a fiber optic fluorosensor in which the magnitude of the fluorescence emitted by a dye (e.g. perylenedibutryate) can be modulated by the level of humidity. Another mode of FO humidity sensing is to take advantage of materials that change refractive index based on changes in humidity. For example, plastic fiber optic coated may be coated with hydroxyethylcellulose, which swells and changes refractive index in the presence of humidity. Interferometric FO sensors measure the phase difference of two light waves from the same source is compared after reflection from a mirror. Some known FO humidity sensors use Mach-Zehnder, Sagnac, or Fabry-Perot interferometric configurations.
In-Fiber Grating Sensors:
In-fiber grating sensors include long period gratings (LPG) and fiber Bragg gratings (FBG). Both the LPG and the FBG possess a section of optical fiber core inscribed with periodic perturbations (grating) of refractive index. The difference between the LPG and FBG is the length of the grating (LPG: 100 μm to 1 mm, FBG: <1 μm). Changes in the cladding of an LPG sensor lead to changes in its transmission spectrum. This property has been used to develop several humidity and liquid-moisture LPG sensor. In order to measure parameters such as humidity, a mechanism must be used to translate humidity into one of the aforementioned physical parameters. Some have accomplished this by using humidity sensitive polymers. For example, a polymer may be coated over the FBG to measure the moisture content of soil. Swelling of the polymer due to absorption of moisture induced a longitudinal strain in the FBG, thus shifting the wavelength and signaling the level of humidity. The sensor had a linear response between 0 to 100% humidity with a response time limited by the swelling rate of the polymer. Alternatively, an FBG may be coated with polyimide, for which moisture-induced swelling produced tension of the FBG. Other FBG-based humidity sensors have a similar construction (polymer-induced straining of the FBG), with some modified for sensing salinity or other analytes. The aforementioned FBG sensors may utilize a thin coating of a water-swellable polymer to induce strain on the FBG, which require a complicated bonding process.
In contrast, an improved FBG sensor and method provides a separate, but adjacent, bead of superabsorbent material is used to absorb liquid moisture. Volumetric expansion of the bead translated to a bending strain of the FBG adjacent to the bead, thus shifting the reflected wavelength. Further, this sensor detects the presence liquid moisture instead of measuring humidity levels.